B1??? represents an integer of from 0 to 3, pB1??? represents an integer of from 0 to 2, and qB1??? represents an integer of from 0 to 4; in a case where a plurality of BB1??? are present, the respective BB1??? may be the same or different; each of RB1 and RB2 which are independent of each other, is a hydrogen atom or an optional substituent; MB1 represents a metal atom, provided that MB1 may take